Add an lts/supported.yml and lts/strict/supported.yml

[dominykas]

This came out of a discussion during nodejs/package-maintenance#379 meeting.

We've agreed in pkgjs that we recommend to bump semver major when dropping support for a specific node version, so that's why the initial proposal in #5 does not automatically remove the versions that people test in.

That said, part of the intent of this repository is to reduce the maintenance burden and so changing the gte-N import could be considered a minor hassle. We might want to offer a config which you simply import, and it automatically drops the node versions that are no longer maintained (possibly with a delay?).

I know this could be a contentious discussion, so I'm splitting this out of #5.